a. Field of the Invention
This invention relates generally to wet flue gas scrubbers, and, more particularly, to a method and apparatus for regenerating flue gas scrubber purge solution into NaOH caustic while removing sulfur containing waste from the plant effluent.
b. Background
Responding to environmental regulations, many oil refiners are currently operating or installing wet flue gas scrubbers to remove SO2 and/or particulates from various waste gas streams. Scrubbers can be used on various refinery processes including coke calciners, Phillips SZorb process, and Fluidized Catalytic Crackers (FCC). Most notably wet flue gas scrubbers (FGS) are used on the regenerator stack of a FCC to remove SO2 and particulates from the combustion gas before expelling it to the atmosphere. To efficiently remove the SO2, caustic soda (or “caustic”, NaOH) must be added to the circulating aqueous solution to control pH in the neutral to slightly basic range. Some portion of the circulating solution must be purged to maintain SO2 removal and control solids content of the solution.
The purge stream is generally further treated in a purge treatment unit (PTU) to remove solids and reduce the chemical oxygen demand (COD) of the solution. After this stage of treatment the liquid effluent is discharged through a waste water treatment plant out the process plant effluent discharge usually into a body of water. The predominate salt in the PTU effluent is sodium sulfate (NaSO4), currently considered benign in salt water effluent zones but of more concern in fresh water or brackish effluent zones. Legal action has been taken against refiners in Illinois and Delaware to mitigate salt in or anticipated to be in refinery effluent due to a FGS operation.
Make-up caustic for the FGS is purchased and shipped into the refinery by truck or railcar. Regenerating the purge from a FGS would greatly reduce caustic purchases. Most caustic is manufactured by an electrolysis separation of NaCl, an electricity intensive process. Recycling caustic will save electricity and benefit the environment versus manufacturing fresh caustic for FGS operations.
Paramount Limited's (India) Double Alkali system uses lime to partially regenerate a caustic-based scrubber purge so that it can be recirculated to the scrubber. However, this process requires a very large purge that would be problematic when dealing with a PTU to remove suspended solids such as from an FCC scrubber. In the double alkali system, calcium sulfite is a produced solid along with calcium sulfate (gypsum) reducing the value of the recovered solid and making settling and filtering more difficult.
Lime or limestone is commonly used in power plants to remove SO2 from flue gas. Direct lime scrubbers are difficult to operate since lime is not very soluble in water and must be circulated in the scrubber as a slurry unlike the easier to operate caustic FGS. Like the process in this invention, the sulfur is removed by a lime scrubber as CaSO4 or gypsum. The gypsum from power plant scrubbers is increasingly being sold into the synthetic gypsum market and used for wall board or portland cement manufacturing. It is anticipated that gypsum made by applying the present invention will be suitable for use as synthetic gypsum.